fallacy

noun

fal·​la·​cy ˈfa-lə-sē How to pronounce fallacy (audio)
plural fallacies
1
a
: a false or mistaken idea
popular fallacies
prone to perpetrate the fallacy of equating threat with capabilityC. S. Gray
b
: erroneous character : erroneousness
The fallacy of their ideas about medicine soon became apparent.
2
a
: deceptive appearance : deception
b
obsolete : guile, trickery
3
: an often plausible argument using false or invalid inference

Did you know?

Philosophers are constantly using the word fallacy. For them, a fallacy is reasoning that comes to a conclusion without the evidence to support it. This may have to do with pure logic, with the assumptions that the argument is based on, or with the way words are used, especially if they don't keep exactly the same meaning throughout the argument. There are many classic fallacies that occur again and again through the centuries and everywhere in the world. You may have heard of such fallacies as the "ad hominem" fallacy, the "question-begging" fallacy, the "straw man" fallacy, the "slippery slope" fallacy, the "gambler's" fallacy, or the "red herring" fallacy. Look them up and see if you've ever been guilty of any of them.

Examples of fallacy in a Sentence

The fallacy of their ideas about medicine soon became apparent. the once-common fallacy that girls just weren't any good at math
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
Burden Of Proof Unlike many of the other fallacies, the burden of proof fallacy doesn’t serve to conjure up false narratives, nor to ignore or refute the points that have been made. Mark Travers, Forbes.com, 1 June 2025 These are straw man fallacies, falsely implying that acknowledging and celebrating the full range of backgrounds, identities and family structures kids bring to the classroom will somehow crowd out instruction time. Heidi Stevens, Chicago Tribune, 30 May 2025 So, there is an economic fallacy here, and the fallacy is that trade deficits actually mean anything. ABC News, 18 May 2025 The governor and the backers of HSR are willfully ignorant of how their toxic embrace of the sunk cost fallacy is harming California. Jon Coupal, Oc Register, 11 May 2025 See All Example Sentences for fallacy

Word History

Etymology

Latin fallacia, from fallac-, fallax deceitful, from fallere to deceive

First Known Use

15th century, in the meaning defined at sense 2b

Time Traveler
The first known use of fallacy was in the 15th century

Cite this Entry

“Fallacy.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/fallacy. Accessed 10 Jun. 2025.

Kids Definition

fallacy

noun
fal·​la·​cy ˈfal-ə-sē How to pronounce fallacy (audio)
plural fallacies
1
: a false or mistaken idea
2
: the quality or state of being false

More from Merriam-Webster on fallacy

Last Updated: - Updated example sentences
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!